QCD Coherence and the Top Quark Asymmetry

TL;DR

This work shows that QCD coherence in parton showers can generate forward–backward asymmetries in top-quark pair production even when the LO process is symmetric. By comparing fixed-order results with multiple LO shower implementations (HERWIG++, PYTHIA, SHERPA), it identifies how soft and recoil-driven radiation, color-flow, and Sudakov factors produce both a negative A_FB at nonzero p_T and a positive inclusive A_FB that grows with the tt̄ invariant mass. The authors highlight the role of recoil strategies and dipole kinematics in shaping differential asymmetries and demonstrate sizable model dependence across generators. These insights have practical implications for interpreting experimental measurements and for refining higher-order matching and shower models, especially in the context of LHC charge asymmetries. Overall, the paper stresses that recoil and coherence effects in parton showers capture essential physics beyond fixed-order calculations, guiding future theoretical and experimental analyses of tt̄ asymmetries.

Abstract

Coherent QCD radiation in the hadroproduction of top quark pairs leads to a forward--backward asymmetry that grows more negative with increasing transverse momentum of the pair. This feature is present in Monte Carlo event generators with coherent parton showering, even though the production process is treated at leading order and has no intrinsic asymmetry before showering. In addition, depending on the treatment of recoils, showering can produce a positive contribution to the inclusive asymmetry. We explain the origin of these features, compare them in fixed-order calculations and the Herwig++, Pythia and Sherpa event generators, and discuss their implications.